1. Field of the Invention
The present invention relates to a mobile communication system and, in particular, to a vertical handover method in a heterogeneous overlay network.
2. Description of the Related Art
In multiple access wireless communication systems, a mobile terminal is handed over from one cell to another to maintain a radio connection with the network. Typically, a handover occurring between different access nodes that use the same technology is referred to as a horizontal handover. Unlike the horizontal handover, a vertical handover occurs between different access nodes that use different technologies. Nowadays, there exist various cellular communication network technologies such as a Universal Mobile Telecommunication Service (UMTS), a Code Division Multiple Access 2000 (CDMA2000), a Global System for Mobile communication (GSM), and a General Packet Radio Services (GPRS), and Internet Protocol (IP) based networks such as a World Interoperability for Microwave Access (WiMAX), a Wireless Local Area Network (WLAN), and a Wireless Personal Area Network (WPAN). In order to support interoperability between such heterogeneous communication networks, various vertical handover technologies have been developed.
Unlike the conventional horizontal handover in which a handover is determined on the basis of the location of the mobile terminal, the vertical handover is determined based on of various decision factors as well as the location of the mobile terminal.
Typically, the location of the mobile terminal is estimated by measuring a Received Signal Strength (RSS) transmitted by a base station. In this case, the handover is determined when a value of the RSS is less than a predetermined threshold value.
Another approach for determining a handover is to use a beacon frame, in which the handover is determined when a beacon is received from a neighbor base station. In this case, the mobile terminal determines that it has entered another network, when a new beacon frame is received.
Since different types of networks are overlaid in the heterogeneous network environment, various factors for determining the handover are required in addition to a determination based on the RSS. The handover determination factors include a required service characteristic, a network utilization cost, a network state, a mobile state, and a user preference. The service characteristic refers to service stability, a propagation delay, and data transmission, and the network utilization cost means a charge accounting method. The network state is represented by types of traffic, available bandwidth, packet loss, and the like; and the mobile state refers to a moving pattern, moving speed, and mobility information. The user preference is a factor that can be included by a user's request.
In the location-based vertical handover, the handover determination is made only on the basis of the location of the mobile terminal without taking into consideration the network complexity and the types of traffic of the heterogeneous network overlay environment, resulting in inefficient network resource management.
Also, the conventional vertical handover methods are designed without taking into consideration the service quality required by users and the charge accounting method, divesting the user of chance for saving communication costs.
Also, the various decision factors-based conventional vertical handover methods are designed under a lot of assumptions or require restructuring the current networks, resulting in additional infrastructure costs.
Furthermore, the conventional vertical handover methods are disadvantageous in that the mobile terminal need to have multiple network interfaces for supporting the vertical handover, and collect information on all of the networks, resulting in increases of the overall manufacturing costs and in power consumption.